IL-4 is the main differentiation factor of naive CD4+ T cells into effector Th2 cells, but effector Th2 cells are also the main source of IL-4. This paradigm has not yet been resolved. We have shown that IL-6 derived from antigen presenting cells promote the differentiation of naive CD4+ T cells into effector Th2 cells, although this process requires the presence of endogenous IL-4 produced by CD4 + T cells. We have recently demonstrated that IL-6 induces the early expression of IL-4 gene in CD4 + T cells during activation and this IL-4 further promotes Th2 differentiation. IL-6 induces IL-4 gene expression by upregulating NFATc2 gene and protein levels in naive CD4 + T cells and increasing the NFAT-mediated transcription. The effect of IL-6 is not mimicked with IL-4 and other cytokines. In correlation, specific inhibition of NFAT in CD4+ T cells blocks IL-6-induced Th2 differentiation, but it has little effect on IL-4-induced Th2 differentiation. Unlike naive CD4 +T cells, memory CD4+T cells produce high levels of IL-4 very rapidly in response to antigen stimulation even in the absence of exogenous IL-4. Our Preliminary Studies have revealed that NFAT trascriptional activity is highly and rapidly induced in memory CD4 + T cells. Moreover, we have found that memory CD4 + T cells contain high levels of NFATc2 and NFATc1, while naive CD4 + T cells contain very low levels of NFAT members prior to activation. Thus, the level of NFAT proteins represents another regulatory mechanism for NFAT-mediated transcription. We hypothesize that antigen stimulation alone is not sufficient to induce IL-4 gene expression in naive CD4 + T cells due to the low levels of NFAT proteins in these cells. APC-derived IL-6 increases the expression of NFATc2 gene through activation of C/EBP in naive CD4 + T cells, resulting in increased levels of NFAT-mediated transcription and upregulation of IL-4 gene expression during the activation of these cells. However, the high levels of NFATc1 and NFATc2 already present in memory CD4 + T cells prior to antigen stimulation permit TcR-signals to rapidly induce IL-4 gene expression by NFAT in the absence of exogenous cytokines. To address this hypothesis we will follow these specific aims. Specific Aim 1, to demonstrate that IL-6 does not increase NFAT activity and IL-4 production in memory CD4 + T cells due to the high levels of NFAT proteins already present in these cells. Specific Aim 2, to demonstrate that NFAT is required for TcR-induced IL-4 production in memory CD4 + T cells. Specific Aim 3, To demonstrate that IL-6 induces the expression of NFATc2 gene by activating the C/EBP transcription factor.